Visual display means incorporating loudspeakers

ABSTRACT

A visual display unit having a display screen, a housing in which the display screen is mounted, and a loudspeaker in the housing comprising a member capable of sustaining and propagating input vibrational energy by a plurality of resonant bending wave modes in an operative area, the frequencies of the resonant bending wave modes being interleaved so that resonant bending wave modes are evenly distributed. A transducer is mounted wholly and exclusively on the operative area of the member at a preferential location which substantially maximizes acoustic coupling to the resonant bending wave modes in the member, the resonant bending wave modes in turn producing an acoustic output.

This application is a continuation of application Ser. No. 09/011,834,filed May 13, 1998 now U.S. Pat. No. 6,169,809 filed Sep. 2, 1996 asPCT/GB96/02151, which is a continuation-in-part of application Ser. No.08/707,012, filed Sep. 3, 1996.

TECHNICAL FIELD

The invention relates to visual display apparatus and more particularlyto visual display means (hereinafter visual display units) incorporatingloudspeakers.

BACKGROUND ART

It is known from GB-A-2262861 to suggest a panel-form loudspeakercomprising:

a resonant multi-mode radiator element being a unitary sandwich panelformed of two skins of material with a spacing core of transversecellular construction, wherein the panel is such as to have ratio ofbending stiffness (B), in all orientations, to the cube power off panelmass per unit surface area (μ) of at least 10;

a mounting means which supports the panel or attaches to it a supportingbody, in a free undamped manner;

and an electromechanical drive means coupled to the panel which servesto excite a multi-modal resonance in the radiator panel in response toan electrical input within a working frequency band for the loudspeaker.

DISCLOSURE OF INVENTION

Embodiments of the present invention use members of nature, structureand configuration achievable generally and/or specifically byimplementing teachings of our co-pending parent application Ser. No.08/707,012. Such members thus have capability to sustain and propagateinput vibrational energy by bending waves in operative area(s) extendingtransversely of thickness often but not necessarily to edges of themember(s); are configured with or without anisotropy of bendingstiffness to have resonant mode vibration components distributed oversaid area(s) beneficially for acoustic coupling with ambient air; andhave predetermined preferential locations or sites within said area fortransducer means, particularly operationally active or moving part(s)thereof effective in relation to acoustic vibrational activity in saidarea(s) and signals, usually electrical, corresponding to acousticcontent of such vibrational activity. Uses are envisaged in co-pendingparent application Ser. No. 08/707,012 for such members as or in“passive” acoustic devices without transducer means, such as forreverberation or for acoustic filtering or for acoustically “voicing” aspace or room; and as or in “active” acoustic devices with transducermeans, such as in a remarkably wide range of sources of sound orloudspeakers when supplied with input signals to be converted to saidsound, or in such as microphones when exposed to sound to be convertedinto other signals.

This invention is particularly concerned with active acoustic devicese.g. in the form of loudspeakers for visual display means. Members asabove are herein called distributed mode radiators and are intended tobe characterised as in the above co-pending parent application and/orotherwise as specifically provided herein.

The invention is a visual display unit comprising a display screen, ahousing in which the display screen is mounted and a loudspeaker in thehousing, characterised in that the loudspeaker comprises a a stifflightweight member having capability to sustain and propagate inputvibrational energy by bending waves in at least one operative areaextending transversely of thickness to have resonant mode vibrationcomponents distributed over said at least one area and havepredetermined preferential locations or sites within said area fortransducer means and having a transducer mounted wholly and exclusivelyon said member at one of said locations or sites to vibrate the memberto cause it to resonate forming an acoustic radiator which provides anacoustic output when resonating. The radiator may be integral with thehousing. The housing may have an external wall with which the radiatoris integral. The radiator may comprise a cellular core sandwiched byskin layers, one of which skin layers is integral with the housing. Thesaid one skin layer may be thinner than the average wall thickness ofthe housing. The external wall of the housing may be formed with agroove which surrounds the radiator and which defines a resilientsuspension coupling the loudspeaker to the housing. At least one slotmay be provided in the groove to increase the compliance of thesuspension.

BRIEF DESCRIPTION OF DRAWINGS

The invention is diagrammatically illustrated, by way of example, in theaccompanying drawings, in which:

FIG. 1 is a diagram showing a distributed-mode loudspeaker as describedand claimed in our co-pending parent application Ser. No. 08/707,012;

FIG. 2a is a partial section on the line A—A of FIG. 1;

FIG. 2b is an enlarged cross-section through a distributed mode radiatorof the kind shown in FIG. 2a and showing two alternative constructions;

FIG. 3 is a perspective diagram of an embodiment of visual display unitincorporating a loudspeaker according to the present invention, and

FIG. 4 is a partial cross-sectional view, through the loudspeaker shownin FIG. 3.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 of the drawings, there is shown a panel-formloudspeaker (81) of the kind described and claimed in our co-pendingparent application Ser. No. 08/707,012 comprising a rectangular frame(1) carrying a resilient suspension (3) round its inner periphery whichsupports a distributed mode sound radiating panel (2). A transducer (9)e.g as described in detail with reference to our co-pending applicationSer. Nos. 09/011,773, 09/011,70 and 09/011,831 is mounted wholly andexclusively on or in the panel (2) at a predetermined location definedby dimensions x and y, the position of which location is calculated asdescribed in our co-pending parent application Ser. No. 08/707,012, tolaunch bending waves into the panel to cause the panel to resonate toradiate an acoustic output.

The transducer (9) is driven by a signal amplifier (10), e.g. an audioamplifier, connected to the transducer by conductors (28). Amplifierloading and power requirements can be entirely normal, similar toconventional cone type speakers, sensitivity being of the order of 86-88dB/watt under room loaded conditions.

Amplifier load impedance is largely resistive at 6 ohms, power handling20-80 watts. Where the panel core and/or skins are of metal, they may bemade to act as a heat sink for the transducer to remove heat from themotor coil of the transducer and thus improve power handling.

FIGS. 2a and 2 b are partial typical cross-sections through theloudspeaker (81) of FIG. 1. FIG. 2a shows that the frame (1), surround(3) and panel (2) are connected together by respective adhesive-bondedjoints (20). Suitable materials for the frame include lightweightframing, e.g. picture framing of extruded metal e.g. aluminium alloy orplastics. Suitable surround materials include resilient materials suchas foam rubber and foam plastics. Suitable adhesives for the joints (20)include epoxy, acrylic and cyano-acrylate etc. adhesives.

FIG. 2b illustrates, to an enlarged scale, that the panel (2) is a rigidlightweight panel having a core (22) e.g. of a rigid plastics foam (97)e.g. cross linked polyvinylchloride or a cellular matrix (98) i.e. ahoneycomb matrix of metal foil, plastics or the like, with the cellsextending transversely to the plane of the panel, and enclosed byopposed skins (21) e.g. of paper, card, plastics or metal foil or sheet.Where the skins are of plastics, they may be reinforced with fibres e.g.of carbon, glass, Kevlar (RTM) or the like in a manner known per se toincrease their modulus.

Envisaged skin layer materials and reinforcements thus include carbon,glass, Kevlar (RTM), Nomex (RTM) i.e. aramid etc. fibres in various laysand weaves, as well as paper, bonded paper laminates, melamine, andvarious synthetic plastics films of high modulus, such as Mylar (RTM),Kaptan (RTM), polycarbonate, phenolic, polyester or related plastics,and fibre reinforced plastics, etc. and metal sheet or foil.Investigation of the Vectra grade of liquid crystal polymerthermoplastics shows that they may be useful for the injection mouldingof ultra thin skins or shells of smaller size, say up to around 30 cmdiameter. This material self forms an orientated crystal structure inthe direction of injection, a preferred orientation for the goodpropagation of treble energy from the driving point to the panelperimeter.

Additional such moulding for this and other thermoplastics allows forthe mould tooling to carry location and registration features such asgrooves or rings for the accurate location of transducer parts e.g. themotor coil, and the magnet suspension. Additionally with some weakercore materials it is calculated that it would be advantageous toincrease the skin thickness locally e.g. in an area or annulus up to150% of the transducer diameter, to reinforce that area and beneficiallycouple vibration energy into the panel. High frequency response will beimproved with the softer foam materials by this means.

Envisaged core layer materials include fabricated honeycombs orcorrugations of aluminium alloy sheet or foil, or Kevlar (RTM), Nomex(RTM), plain or bonded papers, and various synthetic plastics films, aswell as expanded or foamed plastics or pulp materials, even aerogelmetals if of suitably low density. Some suitable core layer materialseffectively exhibit usable self-skinning in their manufacture and/orotherwise have enough inherent stiffness for use without laminationbetween skin layers. A high performance cellular core material is knownunder the trade name ‘Rohacell’ which may be suitable as a radiatorpanel and which is without skins. In practical terms, the aim is for anoverall lightness and stiffness suited to a particular purpose,specifically including optimising contributions from core and skinlayers and transitions between them.

Several of the preferred formulations for the panel employ metal andmetal alloy skins, or alternatively a carbon fibre reinforcement. Bothof these, and also designs with an alloy Aerogel or metal honeycombcore, will have substantial radio frequency screening properties whichshould be important in several EMC applications. Conventional panel orcone type speakers have no inherent EMC screening capability.

In addition the preferred form of piezo and electro dynamic transducershave negligible electromagnetic radiation or stray magnetic fields.Conventional speakers have a large magnetic field, up to 1 metre distantunless specific compensation counter measures are taken.

Where it is important to maintain the screening in an application,electrical connection can be made to the conductive parts of anappropriate distributed mode acuostic radition panel or an electricallyconductive foam or similar interface may be used for the edge mounting.

The suspension (3) may damp the edges of the panel (2) to preventexcessive edge movement of the panel. Additionally or alternatively,further damping may be applied, e.g. as patches, bonded to the panel inselected positions to damp excessive movement to distribute resonanceequally over the panel. The patches may be of bitumen-based material, ascommonly used in conventional loudspeaker enclosures or may be of aresilient or rigid polymeric sheet material. Some materials, notablypaper and card, and some cores may be self-damping. Where desired, thedamping may be increased in the construction of the panels by employingresiliently setting, rather than rigid setting adhesives.

Effective said selective damping includes specific application to thepanel including its sheet material of means permanently associatedtherewith. Edges and corners can be particularly significant fordominant and less dispersed low frequency vibration modes of panelshereof. Edge-wise fixing of damping means can usefully lead to a panelwith its said sheet material fully framed, though their corners canoften be relatively free, say for desired extension to lower frequencyoperation. Attachment can be by adhesive or self-adhesive materials.Other forms of useful damping, particularly in terms of more subtleeffects and/or mid- and higher frequencies can be by way of suitablemass or masses affixed to the sheet material at predetermined effectivemedial localised positions of said area.

An acoustic panel as described above is bi-directional. The sound energyfrom the back is not strongly phase related to that from the front.Consequently there is the benefit of overall summation of acoustic powerin the room, sound energy of uniform frequency distribution, reducedreflective and standing wave effects and with the advantage of superiorreproduction of the natural space and ambience in the reproduced soundrecordings.

While the radiation from the acoustic panel is largely non-directional,the percentage of phase related information increases off axis. Forimproved focus for the phantom stereo image, placement of the speakers,like pictures, at the usual standing person height, confers the benefitof a moderate off-axis placement for the normally seated listeneroptimising the stereo effect. Likewise the triangular left/rightgeometry with respect to the listener provides a further angularcomponent. Good stereo is thus obtainable.

There is a further advantage for a group of listeners compared withconventional speaker reproduction. The intrinsically dispersed nature ofacoustic panel sound radiation gives it a sound volume which does notobey the inverse square law for distance for an equivalent point source.Because the intensity fall-off with distance is much less than predictedby inverse square law then consequently for off-centre and poorly placedlisteners the intensity field for the panel speaker promotes a superiorstereo effect compared to conventional speakers. This is because theoff-centre placed listener does not suffer the doubled problem due toproximity to the nearer speaker; firstly the excessive increase inloudness from the nearer speaker, and then the corresponding decrease inloudness from the further loudspeaker.

There is also the advantage of a flat, lightweight panel-form speaker,visually attractive, of good sound quality and requiring only onetransducer and no crossover for a full range sound from each paneldiaphragm.

FIG. 3 illustrates a visual display unit (137) e.g. a computer monitoror the like having a screen (37) formed in any desired fashion e.g. as acathode ray tube or as a liquid crystal display. The unit (137)comprises a box-like housing (101) having opposed sides (102) each ofwhich is formed to incorporate a multi-mode acoustic radiator (2)generally similar to that described above with reference to FIGS. 1 and2 to form loudspeaker (81).

The housing (101) is moulded from plastics and the opposed sides (102)are moulded with generally rectangular relatively thin rectangularareas, in comparison to the general thickness of the housing, bounded bygrooves (100) to define the radiators (2). These areas (2) are stiffenedon their inner faces with a lightweight core (22) which is backed by aninner skin (21) to form a rigid lightweight multi-mode radiator panel(2) of the kind described above e.g. with reference to FIGS. 1 and 2.The grooves effectively define a resilient suspension (3) of the kindshown in FIGS. 1 and 2 and the surrounding housing (101) forms the frame(1). Slots (not shown) may be provided in the grooves and which piercethrough the housing to increase the compliance of the suspension.

A transducer (9) e.g. of the kind described in detail in our co-pendingapplication Ser. Nos. 09/011,773, 09/011,770, and 09/011,831 is attachedto each panel (2) to launch bending waves into the panels to cause themto resonate to produce an acoustic output.

What is claimed is:
 1. A visual display unit comprising a displayscreen, a housing in which the display screen is mounted, and aloudspeaker in the housing, wherein the loudspeaker comprises: a memberhaving selected values of certain physical parameters which enable themember to sustain and propagate input vibrational energy in apredetermined frequency range by a plurality of resonant bending wavemodes in at least one operative area extending transversely of thicknesssuch that the frequencies of the resonant bending wave modes along atleast two conceptual axes of the operative area are interleaved andspread so that there are substantially minimal clusterings anddisparities of spacings of said frequencies, the member when resonatinghaving at least one site at which the number of vibrationally activeresonance anti-nodes is relatively high; and a transducer mounted whollyand exclusively on the member at one of said sites on the member, thetransducer being capable of vibrating the member in the predeterminedfrequency range to couple to and excite the resonant bending wave modesin the member and cause the member to resonate and produce an acousticoutput.
 2. A visual display unit according to claim 1, wherein themember is integral with the housing.
 3. A visual display unit accordingto claim 2, wherein the housing has an external wall and the member isintegral with the external wall.
 4. A visual display unit according toclaim 3, wherein the member comprises a cellular core sandwiched by skinlayers, one of which skin layers is integral with the housing.
 5. Avisual display unit according to claim 4, wherein the said one skinlayer is thinner than the average wall thickness of the housing.